The development of surgical techniques has made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
One system for non-invasive surgery is the Leksell Gamma Knife® Perfexion system, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and is focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. Each of the sources provides a dose of gamma radiation which is insufficient to damage intervening tissue. However, tissue destruction occurs where the radiation beams from all or some radiation sources intersect or converge, causing the radiation to reach tissue-destructive levels. The point of convergence is hereinafter referred to as the “focus point”.
Treatment planning optimization for radiation therapy, including for example gamma knife radio-surgery, aims at maximizing the dose delivered to the target volume within the patient (e.g. in treatment of tumours) at the same time as the dose delivered to adjacent normal tissues is minimized. In treatment planning optimization, the delivered radiation dose is limited by two competing factors where the first one is delivering a maximum dose to the target volume and the second one is delivering the minimum dose to the surrounding normal tissues.
The treatment planning optimization is a process including optimizing the number of shots being used (i.e. number of doses being delivered), the shot size, the shot time, and the position of the shot. Clearly, the irregularity and size of a target volume greatly influence the number of shots needed and the size of the shots being used to optimize the treatment. Normally, the process includes obtaining a three-dimensional representation of the target (e.g. by non-invasive image capturing by X-ray) for the radiation therapy and filling the target with spheres representing the shots without extending area strongly dosed by radiation greatly outside the target and without limited overlapping between shots). It has been shown that in order to preserve dose homogeneity (even coverage of for example an isodose level of 50%) and in a multi-shot plan, shots should not overlap with each other in a too great extent. Thus, overlapping shots may destroy the desired dose homogeneity inside the target. Further, shots protruding outside the target may result in excessive dose to surrounding normal tissues. This requires, for targets of identical volume yet different shapes, use of small shots for complicated contours (i.e. for targets having an irregular shape) and larger shots for regular shapes. In U.S. Pat. No. 6,201,988 to Bourland et al, such an optimization procedure is disclosed. Medial axis transformation (so called skeletonization) is used to characterize the target shape and to determine the shot parameters (i.e. position, collimator size and weight). According to U.S. Pat. No. 6,201,988, only skeleton points are considered for potential shot positions and the corresponding shot size is provided by the skeletonization. The shots are represented by spheres and are modeled as discs in filling process. The endpoints of the skeleton are used as start-points in the filling process. However, the treatment planning optimization shown in U.S. Pat. No. 6,201,988 may provide treatment plans having a non-optimal distribution of shot sizes, for example, an unnecessary large amount of small shot sizes may be included leading to a large number of shots being used.
Hence, there is a need of more efficient methods for planning the treatment and for optimizing the treatment planning.